2-Mercaptobenzothiazole, and its derivatives, are commercially important in the rubber industry as vulcanization accelerators. Many patents and literature references have been published on the preparation and purification of 2-mercaptobenzothiazole.
One of the methods of purification is the treatment of 2-mercaptobenzothiazole with carbon disulfide, according to U.S. Pat. No. 2,090,233 and U.S. Pat. No. 4,061,046. The former requires several extractions of crude 2-mercaptobenzothiazole with carbon disulfide in order to remove impurities (tars) and by-products such as benzothiazole, diphenylthiourea, etc., while U.S. Pat. No. 4,061,646 requires treatment of the molten (about 200.degree. C.) crude 2-mercaptobenzothiazole with cold carbon disulfide by liquid--liquid extraction, thereby forming a slurry of 2-mercaptobenzothiazole crystals which are then filtered and dried. Considerable time and equipment are required to process the resulting carbon disulfides extract containing impurities, unreacted starting materials, and reusable by-products. Moreover, carbon disulfide is a health hazard and is extremely flammable. Another method of purification as disclosed by U.S. Pat. No. 3,681,371, requires the distillation of the crude molten 2-mercaptobenzothiazole, followed by treatment of the distillate with sodium hydroxide. Yet, another patent, U.S. Pat. No. 2,631,153, teaches an improved method for purifying 2-mercaptobenzothiazole involving its treatment with aqueous alkali, aeration of the alkaline solution, and precipitation with an acid, to obtain a high-purity product. In both of the above-mentioned procedures, the time and apparatus required to carry out the distillation and aeration make the processes costly, time-consuming, and difficult to perform on an individual scale.
In a similar process, U.S. Pat. No. 3,804,846, discloses the purification of crude 2-mercaptobenzothiazole by reaction of an alkaline aqueous solution of 2-mercaptobenzothiazole with an oxidizing agent such as hydrogen peroxide, while air is passed through the alkaline solution at 70.degree. C., and the resulting oxidation products are coagulated on activated carbon, thereby removing the impurities. The above-mentioned disadvantages apply to this process as well, in addition to the fact that as a result of excessive oxidation, the disulfide by-product (2,2'-dithio-bisbenzothiazole) is formed, resulting in less pure 2-mercaptobenzothiazole product.
Other procedures, such as disclosed in U.S. Pat. No. 3,770,759 and U.S. Pat. No. 3,904,638, require the use of organic solvents such as toluene and xylene. In all of these processes, additional equipment and longer time are needed to carry out the purification, while the hazard of toxic gases and fire make them unnecessarily expensive and a menace to the safety of the operators.